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Article

Aluminum Sulfate and Straw Enhance Carbon Sequestration in Saline–Alkali Soils

by
Nan Wang
1,
Xinxin Guo
2,
Jinhua Liu
1,
Lanpo Zhao
1,
Hongbin Wang
1,
Biao Sui
1 and
Xingmin Zhao
1,*
1
College of Resources and Environment, Jilin Agricultural University, Changchun 130118, China
2
Faculty of Engineering and Green Technology, University Tunku Abdul Rahman, Kampar 31900, Malaysia
*
Author to whom correspondence should be addressed.
Agronomy 2025, 15(5), 1200; https://doi.org/10.3390/agronomy15051200
Submission received: 11 April 2025 / Revised: 12 May 2025 / Accepted: 13 May 2025 / Published: 15 May 2025
(This article belongs to the Section Soil and Plant Nutrition)
Browse Figures
Versions Notes

Abstract

:
Soil salinization is closely related to land degradation and is presumed to exert a significant effect on the preservation of soil organic carbon (SOC). However, the salinization-induced changes in SOC accumulation over the application gradient of amendments remain unclear. To evaluate the potential for salinization elimination and C sequestration, incubation experiments with four straw addition levels and six aluminum sulfate (Al3+) gradients were conducted in a soda saline–alkali soil, followed by the analysis of partial least squares path models (PLS-PM). The results showed that combined applications significantly reduced soil salinity and sodicity. The C sequestration performance under coapplications was greater than that under individual applications. The SOC and heavy fraction OC (HFOC) contents under the coapplication of 1.6% Al3+ and 10% straw were greater than those under the individual applications of either 1.6% Al3+ or 10% straw by 231.08% and 149.86%, and 9.70% and 18.78%, respectively. Coapplications significantly increased macroaggregates and aggregate-associated SOC levels. PLS-PM demonstrated that Na+, Ca2+ and HCO3− were important environmental factors associated with C sequestration. Overall, our results suggest that Al3+ and straw enhanced C sequestration by regulating salt ions and increasing soil aggregates and that 10% straw combined with 1.6% Al3+ had a greater effect on soda saline–alkali soil. Our study is highly important for the utilization of saline–alkali land and C sequestration in western Jilin Province.

1. Introduction

Saline–alkali soil is widely used for agriculture [1]. Globally, approximately 17 million km2 of land is affected by excessive salt, and the problem is worsening due to global climate change and human activities, resulting in a USD 27.3 billion loss per year [2,3]. Soil organic carbon (SOC) is the central element for nutrient storage and is slowly released in saline–alkali soils [4]. Reduced levels of OC can lead to reduced soil nutrient availability, directly inhibiting crop growth [5]. Over the past 30 years, salinization has led to a 35% reduction in SOC stocks and a 25% decline in grain productivity in the Yellow River Delta [6]. For every 1 g·kg−1 decrease in OC in the saline soils of the Indo-Gangetic Plain, the wheat yield decreased by 15% [2]. OC is also the “cementing agent” of soil aggregates, and its loss leads to destruction of the soil structure, aggravates the hardening of saline–alkali soil and decreases permeability [7]. Increased salt content has been observed to deteriorate aggregate structures in diverse salt-affected regions worldwide, including the Songnen Plain meadows and sorghum fields in Northeast China [8], the Great Prairie Plains in Central and Western Canada, the northern United States [2], the Indo-Gangetic Plain and approximately 30% of Australia’s land area [9]. Therefore, an in-depth understanding of the loss of OC in saline–alkali soils and their aggregates provides an important scientific basis for overcoming the bottleneck of saline–alkali soil management and realizing sustainable agricultural development.
Salt contents and ion types regulate soil aggregate stability, thereby influencing the preservation of aggregate-associated SOC [10]. Among them, soda saline–alkali soil is located in Northeast China, with carbonate (sodium carbonate) and bicarbonate (sodium bicarbonate) as the main salt types [11]. High concentrations of monovalent cations (e.g., Na+ and K+) in saline–alkali soil lead to a 73% reduction in macroaggregate content compared with low-salt-content soils [12]. Conversely, divalent cations (e.g., Ca2+ and Mg2+) contribute to the binding of clay and humic substances to form microaggregates [10]. The reduction in water-stable aggregate stability and associated SOC in aggregates predominantly lead to the depletion of SOC stocks in saline–alkali soils [4]. Soil aggregates and SOC are closely related to soil salt ions.
Improving and utilizing saline–alkali soil involves eliminating of salt–alkali barriers through physical and chemical improvements, returning straw to the field and applying organic fertilizer to increase OC and fertility levels [13,14]. As a chemical amendment, the application of aluminum sulfate (Al3+) changes the degree of salinization in saline–alkali soils [15]. Moreover, after Al3+ was applied to the soil, aluminum ions produced monomeric and polymeric aluminum through hydrolysis, thus promoting soil colloid aggregation [16]. Previous studies have shown that the application of Al3+ in the reclamation of saline–alkali soil reduces the degree of soil salinization, changes the composition of soil aggregates, increases the proportion of large aggregates, and thus improves the soil structure [12]. In alkaline soils, aluminum mainly dominated Al(OH)3 precipitation, and the risk of phytotoxicity was very low [16]. The application of Al3+ to saline–alkali soil increased corn yields by 61%, and no aluminum toxicity was detected in soil or crops [17]. The dual action of Al3+ results in both physical improvement (hardening) and chemical degradation (alkalization), which are rare features of conventional amendments [12]. The improvement of saline–alkali land by Al3+ is in line with Sustainable Development Goal 15 (Life on Land) to restore degraded ecosystems by reducing soil salinity, improving soil structure and enhancing soil C sequestration [18]. On the basis of controlling the degree of salinization in saline–alkali soils, an appropriate increase in straw could increase SOC in the form of increasing stable C fractions [19]. Straw return is a key measure for soil improvement and sustainable agriculture [20]. During the decomposition stage of straw, a variety of biological and abiotic reactions in the soil can lead to the decomposition of straw into humic acid and complex mixtures [21]. These compounds constitute most of the stable OC, and the process of straw addition significantly improves the soil aggregate stability [9]. The dominant ions in the unimproved saline–alkali soil differed from those in the improved saline–alkali soil [10]. Thus, further research is necessary to understand how the distribution of salt ions in improved saline–alkali soils influences SOC within aggregates of various sizes.
In the process of ameliorating saline–alkali soil in western Jilin Province, Al3+ and corn straw have been widely used [18]. Currently, most studies mainly focus on the individual effects of Al3+ or corn straw on SOC and aggregate-associated SOC [22,23]. However, knowledge of their combined effects on SOC fractions and aggregate composition, especially the regulatory mechanisms of salt ions on SOC accumulation under the combined application of corn straw and Al3+, is limited. To clarify the responses of OC fractions and aggregates to corn straw and Al3+ and determine the appropriate coapplication rates, we conducted an incubation experiment at constant temperature and humidity to provide a reference for practical field applications. Accordingly, we examined the salt ions, SOC fractions, aggregates, and aggregate-associated SOC under different proportions of straw and Al3+ to further explore the relationships among the changes in salt ions and SOC fractions in the process of improving saline–alkali soil with corn straw and Al3+ and analyzed the key salt ions that affect the changes in OC fractions. We predicted that (1) as a combination of corn straw and Al3+ is applied to saline–alkali soil, straw decomposes to form humic acid and complex mixtures, and Al3+ hydrolyzes to form a series of products such as hydroxyaluminum polymers; (2) combined application reduces salinization and increases soil structural stability and OC accumulations in saline–alkali soil; and (3) combined application can improve soil structure and C sequestration by reducing salt ions. The results provide a theoretical basis for improving saline–alkali soil and a method for rapidly improving SOC, which is highly important for the utilization of saline–alkali land in western Jilin Province.

2. Materials and Methods

2.1. The Soil and Materials

The studied soil, which is classified as soda meadow saline–alkali soil, was sampled from the 0–20 cm layer at Lesheng Township, Anguang Town, Da’an city, Jilin Province, on 13 September 2023 (45°56′ N, 123°78′ E). A total of 36 sampling points were defined at a distance of 10 cm, followed by mixing, and 1 kg of soil was obtained by quartering. The soil was subsequently passed through a 2 mm (10 mesh) sieve after it was air-dried. The soil pH, electrical conductivity (EC), soluble salt ions, cation exchange capacity (CEC) and exchangeable Na+ were determined after the soil passed through the sieve.
The soil pH, EC, CEC and exchangeable Na+ content were measured using conventional analytical methods according to the handbook for soil analysis [24,25]. The main chemical properties of the tested soil and details of the determinations are shown in Table 1, and the standard limits of salinization indicators in saline–alkali soil are shown in Table S1 [26]. Soil soluble salt ions (including CO32−, HCO3−, Cl−, SO42−, Ca2+, Mg2+, Na+ and K+) were determined according to methods of soluble salts of soil (LY/T1251-1999 [27]). Among them, the leachate of soil pH, EC and soluble salt ions was C dioxide-free water, and the leachate of these indicators was prepared with a water–soil ratio of 5:1. The Flame photometry is from Aopu Analytical Instrument Co., Ltd., Shanghai, China. The pH meter and conductivity meter are form Instrument & Electrical Scientific Instrument Co., Ltd., Shanghai, China. All the reagents used in the experiment were purchased form Sinopharm Group Chemical Reagent Co., Ltd., Shanghai, China. The field capacity (FC) of the soil was determined by the Wilcox method [24,25]. The FC of the soil was 22% ± 1.00%, and the ESP was 58% ± 1.00%. The soil texture was 25% ± 1.00% sand, 38% ± 1.00% silt and 37% ± 2.00% clay. The exchange sodium percentage (ESP) was an important indicator for the classification, utilization and improvement of saline–alkali soil, and ESP was calculated by the following formula:
E S P = E x c h a n g e a b l e   N a + / C E C × 100
The Al3+ used in this research was industrial-grade. The treatment of corn straw in the cultivation experiment referenced the field-based crop residue management methodologies established by Yang et al. [28] and Ran et al. [29]. Corn straw was dried at 25 °C after being naturally air-dried in the field and crushed through a 1 mm sieve. The pH of the straw was 6.79, and the total K content of the straw was 2.82 g·kg−1. Its OC content was 486.14 g·kg−1, and its C:N ratio was 91.90.

2.2. Experimental Design

A simulated incubation experiment with different amounts of Al3+ and straw was performed, including six Al3+ addition treatments (0%, 0.4%, 0.8%, 1.2%, 1.6%, and 2.0% of the soil mass, respectively) and four straw addition treatments (0%, 5%, 10%, and 15% of the soil mass, respectively). Each treatment contained 3 replicates. The abbreviations for 24 different combining treatments were defined, and the amounts of corn straw and Al3+ applied are shown in Table 2. The soil (500 g) was put into a 1200 mL plastic box and mixed with certain amounts of corn straw and Al3+. After the soil moisture was artificially adjusted to 15% (equivalent to 60% of the FC), its weight was recorded and the samples were placed in a constant-temperature incubator (25 °C) and cultivated for 360 days. During incubation, deionized water was added via the weighing method to maintain the water content of the soil at a constant level. The soil was ultimately air-dried for physical and chemical analysis.

2.3. Analytical Methods

2.3.1. Aggregate Composition

The soil aggregate fractions were separated using the wet-sieving procedure with sieves of 2 mm, 0.25 mm, and 0.053 mm apertures [30]. Then, the proportions of macroaggregates (>2 mm, 0.25–2 mm), microaggregates (0.053–0.25 mm) and silt+clay fractions (<0.053 mm) were subsequently calculated [31]. The aggregate size fractions included large-macroaggregate (>2 mm, 5% ± 0%); small-macroaggregate (0.25–2 mm, 10% ± 1%); microaggregate (0.053–0.25 mm, 32% ± 1%); and silt+clay fractions (<0.053 mm, 53% ± 2%). The mean weight diameter (MWD) was calculated according to the following equation [32]:
M W D = ∑ i = 1 n D i / ∑ i = 1 n W i
where D i and W i represent the average diameter (mm) of the aggregates for each size fraction and the weight proportion of each size fraction of the total weight of the aggregates, respectively.

2.3.2. SOC Groups

The SOC content in the soil after being passed through a 0.2 mm sieve was determined. The SOC in the soil and microaggregates was estimated by the K2Cr2O7-H2SO4 method [33]. The relative density grouping method was adopted to extract the heavy-fraction organic carbon (HFOC) in the soil [25], where 10 g of air-dried soil (<2 mm) was shaken with 25 mL of NaI solution of a density at 1.70 g·cm−3 for 1 h. The supernatant was passed through a Millipore filter (0.45 μm, Merck KGaA, Darmstadt, Germany) after centrifugation and the fraction recovered on the filter was the light fraction (LF) (<1.7 g·cm−3). The soil residue in the centrifuge was extracted again with NaI and the additional LF was collected. The two LF components were combined and dried at 60 °C overnight. The sediment in the centrifuge tubes was washed with distilled water 3 times and freeze-dried; the heavy fraction (HF) (>1.7 g·cm−3) was weighed to determine the LF and HF dry matter contents, and the air-dried soil and C contents were measured by the K2Cr2O7-H2SO4 method. The SOC compound quantity (QC) and compound degree (DC) were calculated according to the following equations [34]:
Q C ( g · k g − 1 ) = H F S W × H F O C C / S W
D C = Q C / S O C × 100 %
where H F S W , H F O C C and S W represent the heavy-fraction soil weight (g), heavy-fraction OC content (g·kg−1) and soil weight (g), respectively.

2.4. Statistical Analysis

All the above tests were carried out in triplicate (n = 3). One-way and two-way ANOVAs were conducted to test the differences (p < 0.05) in the aggregate fractions using SPSS 22.0 statistical software (IBM SPSS Institute Inc., Chicago, IL, USA, 2012). Redundancy analysis (RDA) was used to indicate the relative contributions of the environmental factors to the soil C fractions by Canoco 5.0 for Windows. Origin 2021 software was used to produce column charts. A spearman correlation analysis was constructed in Origin 2021. A ring bar chart was generated via the ‘circlize’ R package (R 4.2.1). A partial least squares path model (PLS–PM) was constructed using the ‘plspm’ R package [35] to infer the potential direct and indirect effects of corn straw, Al3+, salt ions, and aggregates on OC. The quality of the PLS–PM was evaluated by examining the goodness-of-fit (GOF) index, where a value > 0.7 indicates a good overall prediction performance of the model, and by examining the coefficients of determination (R2) of the latent variables. The significance level was set at p < 0.05 for all the statistical analyses unless otherwise stated. All the data are presented as the means and standard errors [35].

3. Results

3.1. Salinization Properties of Saline–Alkali Soils

The contents of Ca2+, Mg2+ and K+ increased as corn straw and Al3+ were applied, whereas the Na+ content exhibited the opposite trend (Figure 1a). Under the same Al3+ application rates, Ca2+, Mg2+ and K+ increased with increasing straw amounts. The Ca2+, Mg2+ and K+ contents under the A2.0S15 treatment were the highest among all the straw treatments and were notably greater than those in the A2.0S0 treatment by 150.09%, 29.40% and 188.33%, respectively. Under the same straw application levels, Ca2+, Mg2+ and K+ also enhanced with increasing Al3+. Compared with those in the A0S15 treatment, the Ca2+, Mg2+ and K+ contents under the A2.0S15 treatment increased by 848.93%, 370.72% and 158.21%, respectively. Moreover, significant differences in Ca2+, Mg2+ and K+ contents were detected only in the 15% straw treatment. In contrast, as the straw application rate increased, the Na+ and HCO3− contents under the same Al3+ application rate decreased (Figure 1a,b). The A2.0S15 treatment resulted in the lowest Na+ and HCO3− contents, which were 21.28% and 61.71% lower than those under the A2.0S0 treatment. Under the same straw application rate, the Na+ and HCO3− contents decreased with increasing Al3+. The Na+ and HCO3− contents under the A2.0S15 treatment were the lowest and were 50.16% and 74.56% lower than those under the A0S15 treatment, respectively. The CO32− content under the A0.8S0 treatment decreased the most, by 88.24%. No CO32− was detected under the A1.2–2.0S0, A0.8–2.0S5, A0.4–2.0S10, or A0–2.0S15 treatments, in which the pH was relatively low. In the present study, compared with those in straw, the changes in salt ions in soda saline–alkali soil were affected mainly by the application of Al3+.

3.2. Effects of Al3+ on Straw C Sequestration

Changes in SOC

The effects of corn straw and Al3+ on the total of SOC content are positive, as shown in Figure 2. When no straw was applied, there were no differences in the SOC content in the saline–alkali soil among the Al3+ treatments. Under the 5% straw treatment, the SOC contents of the Al3+-amended soils were higher under the A1.6S5 and A2.0S5 treatments than under the A0.4S5 treatment by 5.40% and 5.22%, respectively. Under the 10% application rate, the SOC contents of the Al3+ treatment for A1.6S10 and A2.0S10 were significantly greater than that for A0.4S10 by 7.58% and 3.92%, respectively. Under a straw application rate of 15%, the SOC contents of the Al3+-amended soils for the A1.6S15 and A2.0S15 treatments were clearly higher than those for the A0.4S15 treatment by 6.58% and 3.19%, respectively. In the present scenario, the SOC increased as the amount of Al3+ increased to some extent; however, the differences were not significant.
Overall, as the amount of Al3+ increased, the SOC contents when 5%, 10% and 15% of straw were applied to the soils were clearly higher than those of the 0% straw treatment by 117.31%, 220.19% and 291.12%, respectively. There were significant differences among the soils with different straw application rates. Specifically, the SOC content was 47.34% greater in the 10% straw treatment than in the 5% straw treatment, and the SOC content was 22.16% greater in the 15% straw treatment than in the 10% straw treatment. These results further indicate that compared with straw, Al3+ has a greater ability to inhibit the mineralization of OC in saline–alkali soils. After more than 5% straw was applied to the saline–alkali soil, the SOC content reached a maximum under the 1.6% application rate of Al3+, which was the optimum addition amount for Al3+ in this study.

3.3. Changes in Aggregate Composition and OC Distribution

3.3.1. Aggregate Distribution

The soil aggregate distribution without coapplication in the natural saline–alkali soil was as follows (Figure 3): the proportions of the microaggregate (0.053–0.25 mm) and silt+clay fractions (<0.053 mm) were greater than those of the large-macroaggregate (>2 mm) and small-macroaggregate (0.25–2 mm) fractions under the A0S0 treatment.
Under A2.0S0, the treatment without straw, the proportions of the >2 mm and 0.25–2 mm fractions were the highest and were notably greater than those under the A0S0 treatment by 99.11% and 62.55%, respectively. Moreover, compared with the A0S0 treatment, A2.0S0 resulted in significantly lower proportions of the 0.053–0.25 mm and <0.053 mm particle sizes (decrease of 13.74% and 12.26%, respectively). These results indicated that long-term Al3+ addition favors the formation of macroaggregates and weakens the formation of microaggregates.
After straw was added to the saline–alkali soil, the changes in the proportions of macroaggregates (>0.25 mm) were ranked in the following order: 15% straw > 10% straw > 5% straw > 0% straw. Compared with the A0S0 treatment, A0S5, A0S10 and A0S15 increased by 8.93%, 20.54%, 27.23%, and 3.09%, 4.63%, 5.79%, respectively, in the >2 mm and 0.25–2 mm aggregates. The proportions of 0.053–0.25 mm and <0.053 mm under 15% straw application were the lowest for the A0S15 treatment among all the treatments and were lower than those for the A0S0 treatment by 1.61% and 2.46%, respectively. Furthermore, the proportions of macroaggregates were increased with the combined application of straw and Al3+, whereas the proportions of the microaggregate and silt+clay fractions decreased with the combined application, indicating that the combined application resulted in changes in aggregate distribution. Compared with the A0S0 treatment, A2.0S15 exhibited increased proportions of the >2 mm and 0.25–2 mm fractions by 128.89% and 68.73%, respectively. The proportions of 0.053–0.25 mm and <0.053 mm particles decreased by 15.73% and 15.14%, respectively. Overall, the effect of “Straw × Al3+” on the aggregate was more significant.
Furthermore, the MWD was affected by the coapplication of straw and Al3+. The MWD obtained for the A2.0S10 treatment was the highest among all the treatments and was 60.71% greater than that for the A0S0 treatment. In summary, coapplication enhanced soil aggregate stability in saline–alkali soil, and the aggregate stability under the A2.0S10 treatment was still greater than that in the other treatments.

3.3.2. Aggregate-Associated SOC

Most aggregate SOC contents (aggregate C) with different particle sizes increased after the coaddition of straw and Al3+ (Figure 4). However, there were no differences in the SOC of the >2 mm particle size (large-macroaggregate-C) proportion among the combined applications, whereas the SOC of the 0.25–2 mm particle size (small-macroaggregate-C) proportion significantly increased with the application rate. Compared with unamended soil, straw increased the small-macroaggregate-C proportion by 56.03%, the SOC in the 0.053–0.25 mm particle size (microaggregate-C) fraction by 92.84%, and the SOC in the <0.053 mm particle size (silt+clay-C) fraction by 45.41% under the A0S15 treatment, and Al3+ increased the small-macroaggregate C by 202.03%, the microaggregate C by 191.63%, and the silt+clay-C fraction by 167.71% under the A2.0S0 treatment. A significant difference in the aggregate-C fraction was found only in the silt+clay fraction under the different application rates of Al3+.
In the present study, the small-macroaggregate-C and silt+clay-C contents were the highest for A2.0S15 treatment, which were significantly higher than those for the A0S0 treatment by 205.09% and 200.10%, respectively, whereas the microaggregate-C content in A2.0S10 treatment was the highest and was clearly higher than that under the A0S0 treatment by 225.12%. In addition, the microaggregates and silt+clay fractions had the highest OC contents, which may be directly related to the changes in the different OC fractions.

3.4. Associations and Relative Importance

3.4.1. Correlations Among Salinization Properties, Aggregate Characteristics and SOC

To further identify the factors affecting the SOC fractions after the combined application of corn straw and Al3+ in saline–alkali soil, a spearman correlation matrix was constructed from the SOC fraction data and the salinization properties of the soil (Figure 5a). Among the OC indicators, SOC and HFOC in the soil were significantly positively correlated with QC and SOC in the large-macroaggregate (>2 mm) fraction, whereas they were negatively correlated with DC. Among the soil aggregate indicators, >2 mm and small macroaggregates (0.25–2 mm) were significantly positively correlated with CEC, Ca2+, Mg2+, and MWD, but negatively correlated with pH, ESP, Na+, CO32−, and HCO3−. Similar correlations were observed for aggregate C. The SOC at 0.25–2 mm, microaggregates (0.053–0.25 mm), silt+clay fractions (< 0.053 mm) and MWD presented the strongest associations with CEC, followed by Ca2+ and Mg2+, whereas they were negatively correlated with pH, ESP, Na+, CO32−, and HCO3−. In contrast, the 0.053–0.25 mm and <0.053 mm fractions exhibited positive correlations with pH, ESP, Na+, CO32−, and HCO3−, whereas they were negatively correlated with CEC, Ca2+, Mg2+ and MWD.
On the basis of the results of the correlation matrix between the SOC fractions and salinization properties, we combined the soil salinization factors to perform RDA on the SOC, HFOC and aggregate-C contents to explore the influence of each factor on the SOC, HFOC and aggregate-C contents under different straw and Al3+ contents (Figure 5b). The soil salinization properties explained 97.2% of the variations in the SOC, HFOC and aggregate-C contents. Thus, the CEC, pH and ESP were the main environmental factors that affected the SOC, HFOC and aggregate-C. However, in this study, the CEC was determined mainly by Ca2+ and Mg2+, whereas pH and ESP were dominated mainly by CO32− and HCO3−.

3.4.2. Driving Factors of the SOC

The partial least squares path model indicated that all predictor variables explained 89.00% of the variations in SOC (Figure 6). Both corn straw and Al3+ had significantly positive effects on Ca2+, while they negatively regulated Na+ and HCO3−. Ca2+ promoted the formation of large macroaggregates, whereas Na+ and HCO3− suppressed large macroaggregates. In contrast, Na+ also suppressed small aggregates while promoting microaggregates and silt+clay. In addition, the HCO3− in saline–alkali soils promoted microaggregates. In general, both corn straw and Al3+ and all sizes of aggregates promoted saline–alkali SOC, and the promoting effects decreased with decreasing aggregate size. Corn straw, Al3+, Ca2+, microaggregates and large macroaggregates had positive overall effects, whereas Na+, HCO3−, small macroaggregates and silt+clay had negative overall effects on SOC. Large macroaggregates had the greatest positive effect among all the aggregates, whereas small macroaggregates had the greatest negative effect. Thus, macroaggregates were the strongest driving factor of SOC.

4. Discussion

4.1. Reduction in Soil Salinity–Sodality

The reason for the high pH in saline–alkali soil is that the Na+ adsorbed on the soil colloid hydrolyzes and produces OH−, which increases the alkalinity of the soil [1]. H+ produced by the hydrolysis of Al3+ can neutralize OH−, thus reducing the soil pH [22]. However, Al3+ contents above 0.8% did not further alter the pH (Figure S1), as at this concentration, H+ generated by the hydrolysis of Al3+ neutralized all OH− [16]. The decomposition and mineralization of straw release organic acids and CO2, among which CO32− is released and converted into HCO3−, thus gradually reducing CO32− [9]. Moreover, the presence of Al3+ accelerated straw decomposition. A large amount of H+ that was generated by the hydrolysis of Al3+ in saline–alkali soil neutralized CO32− and HCO3− [36]. Therefore, the combined application of Al3+ and straw reduced soil CO32− and HCO3−.
In this study, the application of straw increased the K+, Ca2+, and Mg2+ contents and CEC in saline–alkali soil but decreased the soil ESP. Previous studies have shown that corn straw is rich in nutrients, and that returning straw improves soil K contents [37]. After straw return, K+ was liberated into the soil, increasing the soil K+ content [38]. Straw can generate organic acids that can degrade, convert, and release K, hence increasing the soil K supply capacity [39]. After the decomposition of straw in saline–alkali soil, the organic acids can directly activate solidified Ca2+ by dissolving carbonate [40]. Similarly, Na+ was also replaced with Ca2+, thus contributing to an increase in Ca2+ and a decrease in Na+ [41]. Saline–alkali soil contains many carbonates, and H+ is released by the hydrolysis of Al3+, which promotes the precipitation of Ca2+ and Mg2+ from carbonate to bicarbonate dissolution [42]. Additionally, Ca2+ and Mg2+ were replaced with Na+ adsorbed by the soil colloids [12]. Therefore, Na+ gradually decreased as Al3+ increased (Figure 1a). There were many carboxyl groups, phenolic groups, and phenolic hydroxyl groups in the humus after straw degradation [23]. These functional groups provided more sites for exchangeable ions to absorb to the saline–alkali soil, which resulted in an increase in the CEC and a decrease in the ESP in the saline–alkali soil (Figure S2).

4.2. Enhancement of Soil C Sequestration Performance

Significant increases were found only for SOC and HFOC at the 5% straw application rate (Figure 2 and Figure S3). This might be related to straw addition and decomposition time, indicating that SOC tended to be transformed into a stable HFOC fraction when a small amount of straw was added to saline–alkali soil, whereas increasing the application rate blindly led to a decrease in HFOC [43]. Thus, a longer decomposition time may be needed for SOC to be converted into HFOC.
Under the same straw application rate, the SOC and HFOC increased with increasing Al3+ content. Our previous studies revealed that the application of Al3+ increased the C contents of the humus fractions in saline–alkali soil [22]. This might be attributed to the clay mineral composition of the saline–alkali soil being mainly composed of 2:1 montmorillonite and Al3+ playing a “key bridge connection” role in the binding process of humic acid and montmorillonite [16]. Therefore, the increase in humic acid content also led to increases in SOC and HFOC. In contrast, Al3+ with a 1.6% application rate slightly increased the SOC and HFOC compared with 0% Al3+. This result might be related to Al3+ itself, as the addition of Al3+ to saline–alkali soil was reported to inhibit the mineralization of straw C and promote C sequestration by straw addition [22]. However, the SOC and HFOC slightly decreased when the Al3+ content exceeded 1.6%, indicating that the optimum addition of Al3+ was 1.6%, and its positive effect on the SOC and HFOC contents was not obvious when the application rate was greater than 1.6%.
The concentration, speciation, availability and phytotoxicity of Al are highly dependent on the chemical environment, like the ionic strength and pH of the soil [44]. When the pH is alkaline (pH > 7), the most dominant form of Al is Al(OH)4− (aluminate ion) [45]. The aluminum compound is biologically unavailable to plants and is thus represented as a non-phytotoxic form. In this study, Al3+ of different gradients was added to soda saline–alkali soil [46]. With the hydrolysis of Al3+, the soil pH gradually decreased [22]. However, at the same time, the excessive Al3+ that has not been neutralized with soil OH− also leads to an increase in solubility due to the decrease in pH and continues to exist in the form of trivalent cations (Al3+) [16]. This ionic form of aluminum is extremely toxic and one of the major contributing factors restricting the growth of microorganisms and plants [47]. In conclusion, 2.0% Al3+ is not only detrimental to the decomposition of straw but also to the reduction in SOC content.
In the present study, correlation analysis revealed that there was a very significant positive correlation between SOC and HFOC (Figure 5a), indicating that organic matter in soil gradually participated in organic–inorganic complexes during the process of straw transformation. Overall, QC increased with coapplication rate (Figure S4), which was caused mainly by HFOC. The HFOC was significantly positively correlated with the QC (Figure 5a). Therefore, straw might promote QC by increasing HFOC in this study. Compared with that under the 0% straw treatment, a rapid increase in HFOC was observed under the 5% application rate, thus further leading to an increase in QC. On the other hand, as reported, Ca2+ bonded with the surface of exogenous OC [10]. The combined application of straw and Al3+ significantly increased the Ca2+ content in saline–alkali soil (Figure 1a), which was beneficial for increasing the HFOC content and C sequestration level of the complex, thus increasing the saline–alkali soil QC. The QC increased with increasing application rates when the 1.6% application rate was not exceeded, whereas it slightly decreased when the application rate exceeded 1.6%. This might be attributed to the application rate of more than 1.6% Al3+ reducing the C sequestration by straw addition [48]. In contrast, the DC decreased with increasing straw and Al3+ coapplication rates (Figure S4). Under 1 year of amendment, the decrease in DC might be attributed to SOC. SOC was negatively correlated with DC (Figure 5a), which was similar to the results of Liu et al. [49]. Excessive application of straw in saline–alkali soil failed to convert HFOC in a short period of time, resulting in a decreased proportion of HFOC, thus further leading to a decrease in DC.

4.3. Comprehensive Effects of Straw and Al3+ on the Soil Aggregate Composition

In this study, the addition of Al3+ increased the proportions of soil large macroaggregates and small macroaggregates (Figure 4). This effect occurred because the monomeric aluminum, polymeric aluminum ions and aluminum-hydroxy cations that were generated by the hydrolysis of aluminum ions promoted the coagulation of soil colloids after adding Al3+ to saline–alkali soil, further promoting a greater cohesion of aggregates and thereby maintaining stability and nondispersion [22]. This finding aligns with previous studies demonstrating that macroaggregates have a greater capacity to retain OC and soil nutrients than microaggregates [50]. However, microaggregates exhibited greater stability and durability in retaining OC [32,51]. These findings indicate that the aggregate sizes play a crucial role in controlling the dynamics of soil organic matter, surpassing the protection provided by resistant organic matter molecules [52]. Macroaggregates primarily sequester easily decomposable organic matter through physical protection, whereas microaggregates tend to store more recalcitrant C through chemical protection, ensuring long-term preservation [53]. Moreover, straw addition increased the macroaggregate fraction but decreased the microaggregate and silt+clay fractions (Figure 4). This was in accordance with findings of previous studies [37]. Corn straw residue and the degree of organic matter decomposition were crucial factors in the formation of soil aggregates [20]. The organic acids and humus generated by straw decomposition promoted the formation and stability of large particle aggregates in the soil [54]. Our study revealed that there was a significant interaction between straw addition and the addition of different amounts of Al3+ to the aggregates (Figure 3), indicating that coapplication can obviously promote the formation of soil aggregates, compared with individual application of straw or Al3+. The main reason was that Al3+ provides the necessary bond bridges for the formation of aggregates [16], whereas the humus that formed after straw decomposition connects Al3+ with the soil colloidal clay fractions and provides more adsorption sites [43]. Therefore, the promotion effect of the combination was significantly greater than that of the individual application.

4.4. Effects of Straw and Al3+ on C Sequestration in Soil Aggregates

The protective effect of aggregates on SOC was partly reflected in physical protection, and the improvement in soil aggregate stability significantly strengthened the protection mechanism [55]. Adding straw and Al3+ could enhance the organic matter input process, which promoted soil aggregate stability [43]. This conclusion was consistent with our findings. On the basis of the above analysis, the coaddition of straw and Al3+ improved the soil aggregate stability and OC content to improve the soil stability and maintain and stabilize the SOC pool [55]. Additionally, the interior of microaggregates has been described as inherently oligotrophic, with low nutrient and O2 availability resulting in microbial activity [53]. Soil salinization can affect the soil microbial community and activity by destroying soil macroaggregates [40]. In this study, Al3+ decreased the alkalinity of the soil and improved the distribution of the soil aggregates. It is inferred that an appropriate amount of Al3+ can provide a good microenvironment for the growth of soil microorganisms by improving the structure of aggregates, which is conducive to soil C sequestration to a certain extent [17].
The application of straw and Al3+ can affect the soil C pool and its stability through changing the C distribution in different aggregate fractions. These effects are influenced by the types and application rates of amendments [43,56]. In this study, the combination of straw and Al3+ significantly increased the OC contents in all the aggregates, and the effects improved with increasing application rates. However, there were no differences in the C content of the large-macroaggregate fraction (Figure 4). The advantages of the combined application of straw and Al3+ explain the improved effects of the combination on increasing aggregate-associated SOC. Under the combined application, the SOC increased with decreasing aggregate size from the perspective of aggregate particle size (Figure 4). Thus, SOC was mainly stored in the silt+clay fraction, and its content was also the highest. As reported, microaggregates and silt+clay fractions could provide more protection for SOC than macroaggregates and reduce OC loss [57]. For example, Xu et al. reported that there was a significant positive correlation between large aggregates and the active C fraction, which was unstable (e.g., aromatic C and carbonyl C) [58]. The active C fraction encouraged the formation of large aggregates, whereas the microaggregates were protected by the more stable C fraction [59].

4.5. Regulatory Mechanism of Soil C Sequestration

In addition to straw and Al3+, our study revealed that soil salinity factors were important for SOC (Figure 5b and Figure 6). In the present study, Na+, CO32− and HCO3− were significantly negatively correlated with the macroaggregate, small-macroaggregate-C, microaggregate-C, and silt+clay-C fractions and MWD, whereas they were significantly positively correlated with the microaggregate and silt+clay fractions (Figure 5a). This impact occurred because when monovalent ions such as Na+ occupied many exchange sites, the connection between organic molecules and the soil mineral surface weakened, resulting in weakened physical protection of the SOC by the aggregates [60]. Spearman’s correlation coefficients revealed that Ca2+ and Mg2+ were significantly positively related to the macroaggregate, small-macroaggregate-C, microaggregate-C, and silt+clay-C fractions, whereas they were significantly negatively related to the microaggregate and silt+clay fractions (Figure 5a). These findings indicated that Ca2+ and Mg2+ promoted the formation and stabilization of large aggregates and SOC in the aggregates. Generally, a high CEC suggests a high tendency for soil aggregation [61]. On the one hand, multivalent ions such as Ca2+ and Mg2+ can tightly bind the cations that are adsorbed on the surface of soil particles, maintaining the soil aggregation structure [10]. On the other hand, Ca2+ and Mg2+, especially Ca2+, can combine negatively charged soil clay with SOC, reducing the risk of OC mineralization [60]. Our research indicated that the synergistic effect of Al3+ and straw in saline–alkali soil significantly enhanced the stability and fixation of OC through physical protection (aggregate formation). The straw and Al3+ can promote the formation of soil aggregates by increasing Ca2+ and Mg2+ contents, increasing the protection of OC by aggregates, and ultimately improving C sequestration in saline–alkali soils. Overall, these findings provide valuable insights into the roles of straw and Al3+ in improving C sequestration affected by salinization in saline–alkali regions.

5. Conclusions

In summary, our study underscores the importance of straw and Al3+ in reducing saline–alkali stress and enhancing C sequestration in saline–alkali soil. The combined application of straw and Al3+ to improve saline–alkali soils mainly depends on the decomposition of straw and the hydrolysis of Al3+. The decomposition of straw to form humic acid and complex mixtures increased the SOC content to some extent, whereas the hydrolysis of Al3+ not only reduced the soil salinity but also improved the soil structure. Taken together, these findings support the view that combined applications can promote the formation of soil aggregates by increasing the Ca2+ and Mg2+ contents, increasing the protection of OC by aggregates, and ultimately improving C sequestration in saline–alkali soils. As such, improving the soil aggregate distribution by decreasing Na+ and increasing divalent cations (e.g., Ca2+ and Mg2+) is an indispensable part of the process of enhancing SOC in saline–alkali soils. This study, which is based on the combined application of straw and Al3+ to increase C sequestration in saline–alkali soil, provides a theoretical basis for improving soil degradation. Our findings also contribute to strategies for the sustainable development and C sequestration of saline–alkali land worldwide.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/agronomy15051200/s1, Table S1: The standard limits of salinization indicators in saline–alkali soil; Figure S1: Effect of corn straw and Al3+ combined application on pH in saline–alkali soil; Figure S2: Changes in CEC and ESP in saline–alkali soil responded to the combined application of corn straw and Al3+; Figure S3: The HFOC of saline–alkali soil under the combined application of corn straw and aluminum sulfate; Figure S4: Changes in organic–inorganic compounds in soda saline–alkali soil after combined application with corn straw and Al3+.

Author Contributions

Conceptualization, N.W.; investigation, H.W., B.S. and L.Z.; visualization, N.W.; data curation, N.W.; formal analysis, N.W., B.S., X.G., J.L. and L.Z.; writing—original draft, N.W.; methodology, X.G. and J.L.; writing—review and editing, N.W., X.G., J.L., H.W., B.S. and X.Z.; software, N.W.; supervision, L.Z. and H.W.; project administration, X.Z.; funding acquisition, X.Z. and J.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Key Research and Development Program Project (2024YFD1501005), the Natural Science Foundation of Jilin Province (20210101095JC), and the Chinese Academy of Engineering Cooperation Project (JL2021-19).

Data Availability Statement

The data presented in this study are available on request.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Qu, Y.; Tang, J.; Liu, B.; Lyu, H.; Duan, Y.; Yang, Y.; Wang, S.; Li, Z. Rhizosphere Enzyme Activities and Microorganisms Drive the Transformation of Organic and Inorganic Carbon in Saline–Alkali Soil Region. Sci. Rep. 2022, 12, 1314. [Google Scholar] [CrossRef] [PubMed]
  2. Negacz, K.; Malek, Ž.; De Vos, A.; Vellinga, P. Saline Soils Worldwide: Identifying the Most Promising Areas for Saline Agriculture. J. Arid. Environ. 2022, 203, 104775. [Google Scholar] [CrossRef]
  3. Mukhopadhyay, R.; Sarkar, B.; Jat, H.S.; Sharma, P.C.; Bolan, N.S. Soil Salinity under Climate Change: Challenges for Sustainable Agriculture and Food Security. J. Environ. Manag. 2021, 280, 111736. [Google Scholar] [CrossRef] [PubMed]
  4. Zhang, X.; Heng, J.; Zhao, Y.; Wang, S.; Wang, Y.; Hu, Z. Effect of Salinity on Carbon Sequestration in Constructed Wetlands and Its Functional Mechanisms. Bioresour. Technol. 2024, 391, 129915. [Google Scholar] [CrossRef]
  5. Zhao, X.; Gao, J.; Yu, X.; Borjigin, Q.; Qu, J.; Zhang, B.; Zhang, S.; Li, Q.; Guo, J.; Li, D. Evaluation of the Microbial Community in Various Saline Alkaline-Soils Driven by Soil Factors of the Hetao Plain, Inner Mongolia. Sci. Rep. 2024, 14, 28931. [Google Scholar] [CrossRef]
  6. Yang, C.; Lv, D.; Jiang, S.; Lin, H.; Sun, J.; Li, K.; Sun, J. Soil Salinity Regulation of Soil Microbial Carbon Metabolic Function in the Yellow River Delta, China. Sci. Total Environ. 2021, 790, 148258. [Google Scholar] [CrossRef]
  7. Yudina, A.; Kuzyakov, Y. Dual Nature of Soil Structure: The Unity of Aggregates and Pores. Geoderma 2023, 434, 116478. [Google Scholar] [CrossRef]
  8. Feng, H.; Wang, S.; Gao, Z.; Pan, H.; Zhuge, Y.; Ren, X.; Hu, S.; Li, C. Aggregate Stability and Organic Carbon Stock under Different Land Uses Integrally Regulated by Binding Agents and Chemical Properties in Saline-sodic Soils. Land Degrad. Dev. 2021, 32, 4151–4161. [Google Scholar] [CrossRef]
  9. Huo, Y.; Hu, G.; Han, X.; Wang, H.; Zhuge, Y. Straw-Returning Reduces the Contribution of Microbial Anabolism to Salt-Affected Soil Organic Carbon Accumulation over a Salinity Gradient. Soil. Ecol. Lett. 2023, 5, 220168. [Google Scholar] [CrossRef]
  10. Yang, W.; Song, X.; He, Y.; Chen, B.; Zhou, Y.; Chen, J. Distribution of soil organic carbon density fractions in aggregates as influenced by salts and microbial community. Land 2023, 12, 2024. [Google Scholar] [CrossRef]
  11. Luo, S.; Tian, L.; Chang, C.; Wang, S.; Zhang, J.; Zhou, X.; Li, X.; Tran, L.-S.P.; Tian, C. Grass and Maize Vegetation Systems Restore Saline-Sodic Soils in the Songnen Plain of Northeast China. Land Degrad. Dev. 2018, 29, 1107–1119. [Google Scholar] [CrossRef]
  12. Zhou, M.; Liu, X.; Meng, Q.; Zeng, X.; Zhang, J.; Li, D.; Wang, J.; Du, W.; Ma, X. Additional Application of Aluminum Sulfate with Different Fertilizers Ameliorates Saline-Sodic Soil of Songnen Plain in Northeast China. J. Soils Sediments 2019, 19, 3521–3533. [Google Scholar] [CrossRef]
  13. Wu, L.; Zhang, S.; Chen, M.; Liu, J.; Ding, X. A Sustainable Option: Biochar Addition Can Improve Soil Phosphorus Retention and Rice Yield in a Saline–Alkaline Soil. Environ. Technol. Innov. 2021, 24, 102070. [Google Scholar] [CrossRef]
  14. Jat Baloch, M.Y.; Zhang, W.; Sultana, T.; Akram, M.; Shoumik, B.A.A.; Khan, M.Z.; Farooq, M.A. Utilization of Sewage Sludge to Manage Saline–Alkali Soil and Increase Crop Production: Is It Safe or Not? Environ. Technol. Innov. 2023, 32, 103266. [Google Scholar] [CrossRef]
  15. Chen, X.; Yang, S.; Wen, X.; Guo, F.; Lou, S. Decision of Straw Deep Burial and Aluminum Sulfate Drip Irrigation in Soda Saline Soil Based on Grey Relation Analysis and TOPSIS Coupling. Agronomy 2024, 14, 3. [Google Scholar] [CrossRef]
  16. Anwari, G.; Tianxu, Y.; Alio Moussa, A.; Wentao, Z.; Mandozai, A.; Gamal, M.; El-Rahim, A.; Feng, J. Influence of Biochar and Aluminum Sulfate on Rice Growth and Production in Saline Soil Influence of Biochar and Aluminum Sulfate on Rice Growth and Production in Saline Soil. J. Crop. Improv. 2022, 37, 2151541. [Google Scholar] [CrossRef]
  17. Muhammad, N.; Zvobgo, G.; Zhang, G. A Review: The Beneficial Effect of Aluminum on Plant Growth in Acid Soil and the Possible Mechanisms. J. Integr. Agric. 2018, 17, 60345–60347. [Google Scholar]
  18. Xiao, F.; Zhou, B.; Wang, H.; Duan, M.; Feng, L. China Effects of Different Soil Amendments on Physicochemical Property of Soda Saline-Alkali Soil and Crop Yield in Northeast China. Int. J. Agric. Biol. Eng. 2022, 15, 192–198. [Google Scholar] [CrossRef]
  19. Wang, H.; Chen, Y.; Chen, L.; Liao, B.; Dai, Q.; Zhang, H.; Liao, P. Gypsum Application and Straw Incorporation Interact to Alleviate Methane Emissions in Coastal Saline-Alkali Rice Soils. J. Soil. Sci. Plant Nutr. 2024, 24, 4398–4406. [Google Scholar] [CrossRef]
  20. Che, W.; Piao, J.; Gao, Q.; Li, X.; Li, X.; Jin, F. Response of Soil Physicochemical Properties, Soil Nutrients, Enzyme Activity and Rice Yield to Rice Straw Returning in Highly Saline-alkali Paddy Soils. J. Soil. Sci. Plant Nutr. 2023, 23, 4396–4411. [Google Scholar] [CrossRef]
  21. Zhang, G.; Bai, J.; Zhao, Q.; Jia, J.; Wang, X.; Wang, W.; Wang, X. Soil Carbon Storage and Carbon Sources under Different Spartina Alterniflora Invasion Periods in a Salt Marsh Ecosystem. Catena 2021, 196, 104831. [Google Scholar] [CrossRef]
  22. Zhao, X.; Zhu, M.; Guo, X.; Wang, H.; Sui, B.; Zhao, L. Organic Carbon Content and Humus Composition after Application Aluminum Sulfate and Rice Straw to Soda Saline-Alkaline Soil. Environ. Sci. Pollut. Res. 2019, 26, 13746–13754. [Google Scholar] [CrossRef] [PubMed]
  23. Hu, M.; Qu, Z.; Li, Y.; Xiong, Y.; Huang, G. Contrasting Effects of Different Straw Return Modes on Net Ecosystem Carbon Budget and Carbon Footprint in Saline-Alkali Arid Farmland. Soil. Till Res. 2024, 239, 106031. [Google Scholar] [CrossRef]
  24. Bao, S. Soil Agrochemical Analysis, 3rd ed.; China Agriculture Press: Beijing, China, 2000; ISBN 7-109-06644-4. [Google Scholar]
  25. Lu, R.K. Analysis Method of Soil Agricultural Chemistry; China Agricultural Science and Technology Press: Beijing, China, 2000. [Google Scholar]
  26. Wicke, B.; Smeets, E.; Dornburg, V.; Vashev, B.; Gaiser, T.; Turkenburg, W.; Faaij, A. The Global Technical and Economic Potential of Bioenergy from Salt-Affected Soils. Energy Environ. Sci. 2011, 4, 2669–2681. [Google Scholar] [CrossRef]
  27. LY/T1251-1999; Analysis methods of water soluble salts of forest soil. National Forestry and Grassland Administration: Beijing, China, 1999.
  28. Yang, H.; Xia, J.; Xie, W.; Wei, S.; Cui, Q.; Shao, P.; Sun, J.; Dong, K.; Qi, X. Effects of Straw Returning and Nitrogen Addition on Soil Quality of a Coastal Saline Soil: A Field Study of Four Consecutive Wheat-maize Cycles. Land. Degrad. Dev. 2023, 34, 2061–2072. [Google Scholar] [CrossRef]
  29. Ran, C.; Gao, D.; Bai, T.; Geng, Y.; Shao, X.; Guo, L. Straw Return Alleviates the Negative Effects of Saline Sodic Stress on Rice by Improving Soil Chemistry and Reducing the Accumulation of Sodium Ions in Rice Leaves. Agric. Ecosyst. Environ. 2023, 342, 108253. [Google Scholar] [CrossRef]
  30. Elliott, E.T. Aggregate Structure and Carbon, Nitrogen, and Phosphorus in Native and Cultivated Soils. Soil. Sci. Soc. Am. J. 1986, 50, 627–633. [Google Scholar] [CrossRef]
  31. Xu, C.; Wang, J.; Wu, D.; Li, C.; Wang, L.; Ji, C.; Zhang, Y.; Ai, Y. Optimizing Organic Amendment Applications to Enhance Carbon Sequestration and Economic Benefits in an Infertile Sandy Soil. J. Environ. Manag. 2022, 303, 114129. [Google Scholar] [CrossRef]
  32. Mustafa, A.; Minggang, X.; Ali Shah, S.A.; Abrar, M.M.; Nan, S.; Baoren, W.; Zejiang, C.; Saeed, Q.; Naveed, M.; Mehmood, K.; et al. Soil Aggregation and Soil Aggregate Stability Regulate Organic Carbon and Nitrogen Storage in a Red Soil of Southern China. J. Environ. Manag. 2020, 270, 110894. [Google Scholar] [CrossRef]
  33. Roper, W.R.; Robarge, W.P.; Osmond, D.L.; Heitman, J.L. Comparing Four Methods of Measuring Soil Organic Matter in North Carolina Soils. Soil. Sci. Soc. Am. J. 2019, 83, 466–474. [Google Scholar] [CrossRef]
  34. O’Rourke, S.; Holden, N. Determination of Soil Organic Matter and Carbon Fractions in Forest Top Soils Using Spectral Data Acquired from Visible–near Infrared Hyperspectral Images. Soil. Sci. Soc. Am. J. 2012, 76, 586. [Google Scholar] [CrossRef]
  35. Hengky, L.; Joseph, F.H., Jr.; Richard, N. Partial Least Squares Path Modeling: Basic Concepts, Methodological Issues and Applications; Springer International Publishing: Berlin/Heidelberg, Germany, 2023. [Google Scholar] [CrossRef]
  36. Zhou, Z.; Hua, J.; Xue, J. Salinity Drives Shifts in Soil Microbial Community Composition and Network Complexity along Vegetation Community Succession in Coastal Tidal Flats. Estuar. Coast. Shelf Sci. 2022, 276, 108005. [Google Scholar] [CrossRef]
  37. Jiang, H.; Lv, Q.; Yang, J.; Wang, B.; Dong, H.; Gonsior, M.; Schmitt-Kopplin, P. Molecular Composition of Dissolved Organic Matter in Saline Lakes of the Qing-Tibetan Plateau. Org. Geochem. 2022, 167, 104400. [Google Scholar] [CrossRef]
  38. Zhang, H.; Lu, C.; Pang, H.; Liu, N.; Zhang, X.; Li, Y. Straw Layer Burial to Alleviate Salt Stress in Silty Loam Soils: Impacts of Straw Forms. J. Integr. Agric. 2020, 19, 265–276. [Google Scholar] [CrossRef]
  39. Hou, Y.; Xu, X.; Kong, L.; Zhang, L.; Wang, L. The Combination of Straw Return and Appropriate K Fertilizer Amounts Enhances Both Soil Health and Rice Yield in Northeast China. Agron. J. 2021, 113, 5424–5435. [Google Scholar] [CrossRef]
  40. Liu, M.; Wang, C.; Liu, X.; Lu, Y.; Wang, Y. Saline-Alkali Soil Applied with Vermicompost and Humic Acid Fertilizer Improved Macroaggregate Microstructure to Enhance Salt Leaching and Inhibit Nitrogen Losses. Appl. Soil. Ecol. 2020, 156, 103705. [Google Scholar] [CrossRef]
  41. Wang, S.J.; Chen, Q.; Li, Y.; Zhuo, Y.Q.; Xu, L.Z. Research on Saline-Alkali Soil Amelioration with FGD Gypsum. Resour. Conserv. Recycl. 2017, 121, 82–92. [Google Scholar] [CrossRef]
  42. Wang, T.; Flint, S.; Palmer, J. Magnesium and Calcium Ions: Roles in Bacterial Cell Attachment and Biofilm Structure Maturation. Biofouling 2019, 35, 959–974. [Google Scholar] [CrossRef]
  43. Luo, J.-Q.; Wang, L.-L.; Li, Q.-S.; Zhang, Q.-K.; He, B.-Y.; Wang, Y.; Qin, L.-P.; Li, S.-S. Improvement of hard saline–sodic soils using polymeric aluminum ferric sulfate (PAFS). Soil. Tillage Res. 2015, 149, 12–20. [Google Scholar] [CrossRef]
  44. Bojórquez-Quintal, E.; Escalante-Magaña, C.; Echevarría-Machado, I.; Martínez-Estévez, M. Aluminum, a Friend or Foe of Higher Plants in Acid Soils. Front. Plant Sci. 2017, 8, 1767. [Google Scholar] [CrossRef]
  45. Singh, S.; Tripathi, D.K.; Singh, S.; Sharma, S.; Dubey, N.K.; Chauhan, D.K.; Vaculík, M. Toxicity of Aluminium on Various Levels of Plant Cells and Organism: A Review. Environ. Exp. Bot. 2017, 137, 177–193. [Google Scholar] [CrossRef]
  46. Tyagi, W.; Yumnam, J.S.; Sen, D.; Rai, M. Root Transcriptome Reveals Efficient Cell Signaling and Energy Conservation Key to Aluminum Toxicity Tolerance in Acidic Soil Adapted Rice Genotype. Sci. Rep. 2020, 10, 4580. [Google Scholar] [CrossRef] [PubMed]
  47. Yao, H.; Zhang, S.; Zhou, W.; Liu, Y.; Liu, Y.; Wu, Y. The Effects of Exogenous Malic Acid in Relieving Aluminum Toxicity in Pinus Massoniana. Int. J. Phytoremediat. 2020, 22, 669–678. [Google Scholar] [CrossRef]
  48. Cao, H.; Xie, J.; Hong, J.; Wang, X.; Hu, W.; Hong, J. Organic Matter Fractions Within Macroaggregates in Response to Long-term Fertilization in Calcareous Soil After Reclamation. J. Integr. Agric. 2021, 20, 1636–1648. [Google Scholar] [CrossRef]
  49. Liu, Y.; Liu, W.; Wu, L.; Liu, C.; Wang, L.; Chen, F.; Li, Z. Soil Aggregate-Associated Organic Carbon Dynamics Subjected to Different Types of Land Use: Evidence from 13C Natural Abundance. Ecol. Eng. 2018, 122, 295–302. [Google Scholar] [CrossRef]
  50. Liao, J.; Yang, X.; Dou, Y.; Wang, B.; Xue, Z.; Sun, H.; Yang, Y.; An, S. Divergent Contribution of Particulate and Mineral-associated Organic Matter to Soil Carbon in Grassland. J. Environ. Manag. 2023, 344, 118536. [Google Scholar] [CrossRef]
  51. Yuan, J.; Wang, J.; Ye, J.; Dai, A.; Zhang, L.; Wang, J.; Li, J.; Zhang, M.; Zhang, H.; Chen, D.; et al. Long-Term Organic Fertilization Enhances Potassium Uptake and Yield of Sweet Potato by Expanding Soil Aggregates-Associated Potassium Stocks. Agric. Ecosyst. Environ. 2023, 358, 108701. [Google Scholar] [CrossRef]
  52. Liu, L.; Gunina, A.; Zhang, F.; Cui, Z.; Tian, J. Fungal Necromass Increases Soil Aggregation and Organic Matter Chemical Stability under Improved Cropland Management and Natural Restoration. Sci. Total Environ. 2023, 858, 159953. [Google Scholar] [CrossRef]
  53. Li, J.-Y.; Chen, P.; Li, Z.-G.; Li, L.-Y.; Zhang, R.-Q.; Hu, W.; Liu, Y. Soil Aggregate-Associated Organic Carbon Mineralization and Its Driving Factors in Rhizosphere Soil. Soil. Biol. Biochem. 2023, 186, 109182. [Google Scholar] [CrossRef]
  54. He, W.; Wang, H.; Ye, W.; Tian, Y.; Hu, G.; Lou, Y.; Pan, H.; Yang, Q.; Zhuge, Y. Distinct Stabilization Characteristics of Organic Carbon in Coastal Salt-affected Soils with Different Salinity under Straw Returning. Land. Degrad. Dev. 2022, 33, 4276. [Google Scholar] [CrossRef]
  55. Dai, H.; Chen, Y.; Liu, K.; Li, Z.; Qian, X.; Zang, H.; Yang, X.; Zhao, Y.; Shen, Y.; Li, Z.; et al. Water-Stable Aggregates and Carbon Accumulation in Barren Sandy Soil Depend on Organic Amendment Method: A Three-Year Field Study. J. Clean. Prod. 2019, 212, 393–400. [Google Scholar] [CrossRef]
  56. Cooper, J.; Greenberg, I.; Ludwig, B.; Hippich, L.; Fischer, D.; Glaser, B.; Kaiser, M. Effect of Biochar and Compost on Soil Properties and Organic Matter in Aggregate Size Fractions under Field Conditions. Agric. Ecosyst. Environ. 2020, 295, 106882. [Google Scholar] [CrossRef]
  57. Saha, S.K.; Ramachandran Nair, P.K.; Nair, V.D.; Mohan Kumar, B. Carbon Storage in Relation to Soil Size-Fractions under Tropical Tree-Based Land-Use Systems. Plant Soil. 2010, 328, 433–446. [Google Scholar] [CrossRef]
  58. Xu, L.; Wang, M.; Xie, X.; Shi, X.; Xu, S.; Sun, W.; Shi, Y.; Yu, Q.; Pan, J.; Li, X.; et al. Sensitivity of Soil Aggregation to Soil Organic Carbon Fractions under Land-Use Conversion from Rice to Organic Vegetable Cultivation. Catena 2021, 207, 105661. [Google Scholar] [CrossRef]
  59. Zhang, M.; Song, X.; Wu, X.; Zheng, F.; Li, S.; Zhuang, Y.; Man, X.; Degré, A. Microbial Regulation of Aggregate Stability and Carbon Sequestration under Long-Term Conservation Tillage and Nitrogen Application. Sustain. Prod. Consum. 2024, 44, 74–86. [Google Scholar] [CrossRef]
  60. Setia, R.; Gottschalk, P.; Smith, P.; Marschner, P.; Baldock, J.; Setia, D.; Smith, J. Soil Salinity Decreases Global Soil Organic Carbon Stocks. Sci. Total Environ. 2013, 465, 267–272. [Google Scholar] [CrossRef]
  61. Fungo, B.; Lehmann, J.; Kalbitz, K.; Thionģo, M.; Okeyo, I.; Tenywa, M.; Neufeldt, H. Aggregate Size Distribution in a Biochar-Amended Tropical Ultisol under Conventional Hand-Hoe Tillage. Soil. Till Res. 2017, 165, 190–197. [Google Scholar] [CrossRef]
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Figure 1. Soluble salt of saline–alkali soils in response to the combined application of corn straw and Al3+: (a) Ca2+, (b) Mg2+, (c) Na+, (d) K+, (e) CO32−, and (f) HCO3−. * and ** showed significant and extremely significant correlations at the levels of 0.05 and 0.01, respectively. “ns” indicates no relevance. Capital letters indicate significant differences among Al3+ application under the same straw input (p < 0.05). The different lowercase letters indicate significant differences among straw input amounts under the same Al3+ application (p < 0.05). Error bar represents the standard deviation of mean (n = 3). The innermost circle represents the X-axis and is treated with different amounts of Al3+. The radius represents the Y-axis and represents the concentration of soluble salt ions.
Figure 1. Soluble salt of saline–alkali soils in response to the combined application of corn straw and Al3+: (a) Ca2+, (b) Mg2+, (c) Na+, (d) K+, (e) CO32−, and (f) HCO3−. * and ** showed significant and extremely significant correlations at the levels of 0.05 and 0.01, respectively. “ns” indicates no relevance. Capital letters indicate significant differences among Al3+ application under the same straw input (p < 0.05). The different lowercase letters indicate significant differences among straw input amounts under the same Al3+ application (p < 0.05). Error bar represents the standard deviation of mean (n = 3). The innermost circle represents the X-axis and is treated with different amounts of Al3+. The radius represents the Y-axis and represents the concentration of soluble salt ions.
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Figure 2. The OC in saline–alkali soils under the combined application of corn straw and Al3+. * and ** showed significant and extremely significant correlations at the levels of 0.05 and 0.01, respectively. Capital letters indicate significant differences among straw input amounts under Al3+ application (p < 0.05). The different lowercase letters indicate significant differences among Al3+ application amounts under the same straw input amounts (p < 0.05). Error bar represents the standard deviation of the mean (n = 3).
Figure 2. The OC in saline–alkali soils under the combined application of corn straw and Al3+. * and ** showed significant and extremely significant correlations at the levels of 0.05 and 0.01, respectively. Capital letters indicate significant differences among straw input amounts under Al3+ application (p < 0.05). The different lowercase letters indicate significant differences among Al3+ application amounts under the same straw input amounts (p < 0.05). Error bar represents the standard deviation of the mean (n = 3).
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Figure 3. Variations in aggregate compositions in saline–alkali soil after combined application of corn straw and Al3+. (a) represents 0% straw, (b) represents 5% straw, (c) represents 10% straw, and (d) represents 15% straw. * and ** showed significant and extremely significant correlations at the levels of 0.05 and 0.01, respectively. The purple dots (macroaggregates) represent the sum of the components >2 mm and 0.25–2 mm. Capital letters indicate significant differences among straw input amounts under Al3+ application (p < 0.05). The different lowercase letters indicate significant differences among Al3+ application amounts under the same straw input amounts (p < 0.05). Error bar represents the standard deviation of mean (n = 3).
Figure 3. Variations in aggregate compositions in saline–alkali soil after combined application of corn straw and Al3+. (a) represents 0% straw, (b) represents 5% straw, (c) represents 10% straw, and (d) represents 15% straw. * and ** showed significant and extremely significant correlations at the levels of 0.05 and 0.01, respectively. The purple dots (macroaggregates) represent the sum of the components >2 mm and 0.25–2 mm. Capital letters indicate significant differences among straw input amounts under Al3+ application (p < 0.05). The different lowercase letters indicate significant differences among Al3+ application amounts under the same straw input amounts (p < 0.05). Error bar represents the standard deviation of mean (n = 3).
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Figure 4. The OC distributions in different particle size aggregates of saline–alkaline soil. (a) represents 0% straw, (b) represents 5% straw, (c) represents 10% straw, and (d) represents 15% straw. OCCA represents the OC content of the aggregates. ** showed significant and extremely significant correlations at the levels of 0.01. The different lowercase letters indicate significant differences among Al3+ application amounts under the same straw input amounts (p < 0.05). Error bar represents the standard deviation of the mean (n = 3).
Figure 4. The OC distributions in different particle size aggregates of saline–alkaline soil. (a) represents 0% straw, (b) represents 5% straw, (c) represents 10% straw, and (d) represents 15% straw. OCCA represents the OC content of the aggregates. ** showed significant and extremely significant correlations at the levels of 0.01. The different lowercase letters indicate significant differences among Al3+ application amounts under the same straw input amounts (p < 0.05). Error bar represents the standard deviation of the mean (n = 3).
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Figure 5. (a) Spearman correlation analysis between soil salinization properties and SOC fractions. N = 24; yellow, negative correlation; purple, positive correlation. (b) Ordination diagrams generated by redundancy analysis (RDA) of the effects of environmental factors on SOC, HFOC and aggregate C under the combined application of straw and Al3+.
Figure 5. (a) Spearman correlation analysis between soil salinization properties and SOC fractions. N = 24; yellow, negative correlation; purple, positive correlation. (b) Ordination diagrams generated by redundancy analysis (RDA) of the effects of environmental factors on SOC, HFOC and aggregate C under the combined application of straw and Al3+.
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Figure 6. The partial least squares path models (PLS-PM) illustrating the direct and indirect effects of corn straw and Al3+, Na+, Ca2+, HCO3−, macroaggregate, microaggregate, and silt+clay on SOC (a) and standardized total effects on SOC from PLS-PM (b). The red and blue arrows indicate negative and positive flows of causality, respectively. The numbers on the arrow lines and the thickness of the arrows indicate the normalized path coefficients. The dotted arrows represent nonsignificant path relationships. R2 values next to the latent variables are the coefficients of determination. The GOF index represents the goodness of fit. Asterisks represent significant effects: **, p < 0.01; ***, p < 0.001.
Figure 6. The partial least squares path models (PLS-PM) illustrating the direct and indirect effects of corn straw and Al3+, Na+, Ca2+, HCO3−, macroaggregate, microaggregate, and silt+clay on SOC (a) and standardized total effects on SOC from PLS-PM (b). The red and blue arrows indicate negative and positive flows of causality, respectively. The numbers on the arrow lines and the thickness of the arrows indicate the normalized path coefficients. The dotted arrows represent nonsignificant path relationships. R2 values next to the latent variables are the coefficients of determination. The GOF index represents the goodness of fit. Asterisks represent significant effects: **, p < 0.01; ***, p < 0.001.
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Table 1. The main chemical properties of the tested soil.
Table 1. The main chemical properties of the tested soil.
Basic PropertiesAverage ValueAnalysis Methods
pH10.02 ± 0.35pH meter method (1:5 of soil to water)
EC (mS·cm−1)0.51 ± 0.07Conductivity meter method (1:5 of soil to water)
OC (g·kg−1)8.75 ± 0.82K2Cr2O7-H2SO4 method
CEC (cmol·kg−1)14.25 ± 1.00EDTA–ammonium acetate salt exchange method
Exchange Na+ (cmol·kg−1)8.26 ± 0.01Flame photometry
Soluble K+ (cmol·kg−1)0.02 ± 0.01Flame photometry
Soluble Na+ (cmol·kg−1)4.27 ± 0.18Flame photometry
Soluble Ca2+ (cmol·kg−1)0.63 ± 0.13EDTA titration
Soluble Mg2+ (cmol·kg−1)0.83 ± 0.31EDTA titration
Soluble CO32− (cmol·kg−1)0.35 ± 0.03Double-indicator titration
Soluble HCO3− (cmol·kg−1)0.69 ± 0.05Double-indicator titration
Notes: Values are means ± standard deviation (n = 3).
Table 2. The abbreviations of each treatment and their application amounts of straw and Al3+.
Table 2. The abbreviations of each treatment and their application amounts of straw and Al3+.
The Addition Amount of Al3+ (%)The Addition Amount of Straw (%)
051015
0A0S0A0S5A0S10A0S15
0.4A0.4S0A0.4S5A0.4S10A0.4S15
0.8A0.8S0A0.8S5A0.8S10A0.8S15
1.2A1.2S0A1.2S5A1.2S10A1.2S15
1.6A1.6S0A1.6S5A1.6S10A1.6S15
2.0A2.0S0A2.0S5A2.0S10A2.0S15
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Wang, N.; Guo, X.; Liu, J.; Zhao, L.; Wang, H.; Sui, B.; Zhao, X. Aluminum Sulfate and Straw Enhance Carbon Sequestration in Saline–Alkali Soils. Agronomy 2025, 15, 1200. https://doi.org/10.3390/agronomy15051200

AMA Style

Wang N, Guo X, Liu J, Zhao L, Wang H, Sui B, Zhao X. Aluminum Sulfate and Straw Enhance Carbon Sequestration in Saline–Alkali Soils. Agronomy. 2025; 15(5):1200. https://doi.org/10.3390/agronomy15051200

Chicago/Turabian Style

Wang, Nan, Xinxin Guo, Jinhua Liu, Lanpo Zhao, Hongbin Wang, Biao Sui, and Xingmin Zhao. 2025. "Aluminum Sulfate and Straw Enhance Carbon Sequestration in Saline–Alkali Soils" Agronomy 15, no. 5: 1200. https://doi.org/10.3390/agronomy15051200

APA Style

Wang, N., Guo, X., Liu, J., Zhao, L., Wang, H., Sui, B., & Zhao, X. (2025). Aluminum Sulfate and Straw Enhance Carbon Sequestration in Saline–Alkali Soils. Agronomy, 15(5), 1200. https://doi.org/10.3390/agronomy15051200

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